The present invention relates to an apparatus and method for dispensing a liquid to one or more points of use. More particularly, the present invention relates to such an apparatus and method involving subjecting each of a plurality of chambers to a cycle in which during a fill mode the chambers are filled with the liquid, during a dispense mode the liquid is driven from the chambers to the one or more points of use, and during a return mode unused liquid from the point or points of use is returned to the chambers. Even more particularly, the present invention relates to such an apparatus and method in which the cycle is conducted such that when one chamber is in the dispense mode, another chamber is in the return mode, thereby to allow for the continued circulation of the liquid.
The prior art has proved a number of systems for liquid dispensing that have particular application to the industrial dispensing of process chemicals. One major application of such apparatus is the semiconductor fabrication industry in which chemicals such as photoresist, slurries, and etc. are distributed to one or more points of use such as tools used in such fabrication. In the case of a slurry, the point of use can be a pump employed to feed a polishing tool used in chemical mechanical polishing or planarization.
Although pumps are used for liquid dispensing, it can be important, particularly in the case of semiconductor fabrication, that the chemical be distributed to the points of use without the type of pulsation in flow that can be caused by reciprocating pumping equipment. Such non-pulsating flow can be produced by using pressure vessels to conduct the dispensing. The pressure vessels used for such purpose are pressurized with a chemically non-reactive gas (with respect to the process being conducted) such as ultra-high purity nitrogen. For example, U.S. Pat. No. 5,417,346, has liquid being dispensed from three pressure vessels that are first evacuated to draw in liquid. Thereafter, the pressure vessels are pressurized with nitrogen to dispense the liquid.
In any dispensing system, problems can arise that are associated with the actual usage of the liquid to be dispensed, to wit: not all of the liquid to be dispensed will necessarily be used at the point of use. In order to solve this problem, as set forth in the patent mentioned above, recirculation pathways are provided to recirculate liquid back to a bulk source. The problem can be particularly troublesome in case of slurries because the particles that suspended particles will tend to settle out of the slurry if left stagnant. Additional usage related problems occur where several tools or point of use are fed by one dispensing system. When one or more are taken off-line or brought back on-line, the flow rate of liquid at each of the points of use will change. To this end, in U.S. Pat. No. 5,417,346, the flow rates at points of delivery are sensed to automatically trigger needle valves to assure constant flow conditions. Although not mentioned in the patent cited above, it is common to regulate the entire response of the dispensing system to changes in demand by way of regulating the pressure within the return lines that recirculate liquid back to the bulk sources through pressure regulation valves that regulate liquid pressure. The problem with using valves to assure constant flow conditions is that chemicals to be dispensed can be very corrosive and/or abrasive and as such, valves can act as points of potential wear and maintenance in the dispensing apparatus.
As will be discussed, the present invention provides an apparatus and method for dispensing liquids that incorporates a cyclic operation that inherently allows for continued circulation of the liquid and also, is particularly amenable to controlling flow conditions at the point or points of use without that use of valves that are in contact with the liquid to be dispensed.